The present invention relates to electronic semiconductor devices, and, more particularly, to quantum well devices pumped (clocked or activated) by optical excitation.
Quantum well devices are known in various forms, heterostructure lasers being a good example. Quantum well heterostructure lasers rely on the discrete energy levels in the quantum wells to achieve high efficiency and typically consist of a few coupled quantum wells; see, generally, Sze, Physics of Semiconductor Devices, 729-730 (Wiley Interscience, 2d Ed 1981). High Electron Mobility Transistors (HEMTs) are another type of quantum well device and typically use only one half of a quantum well (a single heterojunction) but may include a stack of a few quantum wells. The HEMT properties arise from conduction parallel to the heterojunctions and in the quantum well conduction or valence subbands; the conduction carriers (electrons or holes) are isolated from their donors or acceptors and this isolation limits impurity scattering of the carriers. See, for example, T. Drummond et al, Electron Mobility in Single and Multiple Period Modulation-Doped (Al,Ga)As/GaAs Heterostructures, 53 J.Appl Phys.1023 (1982). Superlattices consist of many quantum wells so tightly coupled that the individual wells are not distinguishable, but rather the wells become analogous to atoms in a lattice. Consequently, superlattices behave more like new types of materials than as groups of coupled quantum wells; see, generally, L. Esaki et al, Superfine Structure of Semiconductors Grown by Molecular-Beam Epitaxy, CRC Critical Reviews in Solid State Sciences 195 (April 1976).
Conventional quantum wells are typically in the order of 100 .ANG. thick and can easily be fabricated to occupy an area of a few square microns and thus provide extremely small devices with consequent high packing densities and small propagation delays. Fully quantized structures can be as small as 100 .ANG. on a side. However, known quantum well devices are unable to perform the functions of standard electronic components such as flip flops, shift registers, multiplexers, operational amplifiers, random access memories, etc.
Shift registers are standard electronic components with applications in digital computer systems, data-handling systems, serial memory, and control systems and are available in various device technologies such as transistor-transistor logic (TTL), emitter-coupled logic (ECL), and charge coupled devices (CCDs). For example, commercially available serial-in serial-out TTL shift registers include the SN54LS91 (8-bit) and the SN5494 (4-bit). Such TTL devices typically consist of bipolar transistors with dimensions of each transistor of the order of a few microns in all three directions and support shift rates of the order of hundreds of megahertz (propagation delays in the order of nanoseconds). In contrast to TTL devices which rely on switching currents, CCDs shift electron packets by inducing and collapsing adjacent potential wells in a semiconductor substrate. This inducing and collapsing is controlled by applying voltages to electrodes over the semiconductor substrate. CCDs typically have dimensions of a few microns square by a micron deep for each potential well, and the wells are located side by side near the surface of the substrate. CCDs have maximum shift rates on the order of hundreds of megahertz.
However, it is a problem in the known shift registers to decrease size and increase shift rate.